Well I started my critical run-through of KH2. I died twice in Hollow Bastion during the bit where you have to defend the gates. The Samurai got me twice! I made the same stupid mistake. On the third time I avoid and got them at their weakest point. I'm making a point of levelling up after completing each world so I try to stay a bit ahead of the recommended level. I'm also trying to level up drive forms as quickly as possible so I can get the maximum abilities. Only have Valor at the moment but I have high jump level 1 equipped to normal Sora. Need to get Wisdom before I can begin levelling up Valor again!
I'm convinced as I go further along in the game I am going to die.
Leveling up the forms as soon as you get them are optimal seeing as they still require you to level up normally from defeating monsters depending on the form and it helps get those movement abilities faster and upgraded.
From experience, the only parts of the main campaign that were frustrating were both visits to Beast's Castle, the first visit to the Underworld and the battle against Roxas in The World That Never Was. It's mostly the optional battles that will give you more grief than anything else. For most of the main campaign, you should be alright.